NO.sub.2 is an important air pollutant. NO.sub.2 is toxic itself and enters into the complex of chemical reactions which produce photochemical smog. Nitrogen oxides also combine with water to form nitric acid, and contribute significantly to the national problem of "acid rain." NO.sub.x is formed at elevated temperature by endothermic chemical reactions which combine nitrogen with oxygen. Equilibrium concentrations of NO, which forms before NO.sub.2, increase rapidly with temperature, and the kinetics of NO formation is an extremely sensitive increasing function of temperature. The basic kinetics of NO.sub.x formation is fundamentally the same for all combustion processes, wheether they occur in reciprocating piston engines, turbine combustors, or large burners of one or another kind. In every case, the supression of peak flame temperature can radically reduce NO.sub.x outputs in the combustion products.
Total NO.sub.x outputs in the United States were estimated to be as follows in 1975 (Source: Air Pollution, by Henry C. Perkins, McGraw-Hill Book Company, 1974, page 292):
Mobile Sources: 39 percent PA1 Electric Utilities: 23 percent PA1 Pipelines and Gas Plants: 13 percent PA1 Industrial Burners: 19 percent PA1 Household and Commercial: 6 percent
On a world-wide basis human NO.sub.x generation is even more dominated by large burners and other non-automotive sources, with coal combustion constituting 50 percent of world-wide NO.sub.x generation, petroleum refining and other oil burning constituting 28 percent of NO.sub.x emissions, and gasoline combustion only accounting for 14 percent of total world-wide NO.sub.x. (Source: Perkins op. cit., page 293.)
NO.sub.x from gasoline automobiles, which are the dominant mobile NO.sub.x source, can be greatly reduced. For example, the engine design of Automotive Engine Associates described in U.S. Pat. No. 4,344,394, "High Swirl Very Low Pollution Piston Engine Employing Optimizable Vorticity", permits NO.sub.x emissions from gasoline automobiles to be reduced by more than a factor of ten. NO.sub.x emissions from other sources have not been reduced by anything like the same proportion, so in the future it is likely that the NO.sub.x emissions represented by electric utilities, pipelines and gas plants, and industrial burners will become an increasing percentage of total NO.sub.x emissions. In recognition of this, control agencies are tightening NO.sub.x control requirements for these non-automotive sources. The control technology which presently exists for NO.sub.x control from power plants and the other large NO.sub.x sources is expensive and unsatisfactory. It is the purpose of the present invention to produce an inexpensive and very effective means of controlling NO.sub.x from these large sources.
The NO formation process in flames and post-flame gases depends intimately on the temperature-pressure-time history of the individual product of combustion elements. Total NO output from a burner is the integrated NO output from the individual elements. In heterogeneous combustion systems, concentration of NO may vary from element to element within a burner as much as a factor of 1,000 because NO kinetics is such a strong function of chemical species concentrations. Although the kinetics of NO formation is conceptually clear, the computational difficulties of analyzing NO formation in heterogeneous combustion systems are great. Conceptually, it is much easier to think about NO formation in homogeneous combustion systems.
A good description of the kinetics of the NO.sub.x formation process in flames is described in Perkins, op. cit., pages 302-308. The kinetics there described is well-established in the scientific sense, and it is clear that rates of NO formation and equilibrium NO.sub.x concentrations increase very rapidly as temperature increases. Table 12.9 from page 306 of Perkins shows this relation, and is reproduced below.
TABLE 12.9 ______________________________________ Time for NO formation in a gas containing 75 percent nitrogen and 3 percent oxygen Time to form 500 ppm NO concentrations at Temperature, .degree.F. NO, sec equilibrium, ppm ______________________________________ 2400 1,370 550 2800 16.2 1,380 3200 1.10 2,600 3600 0.117 4,150 ______________________________________ Source: AP67.
Time to form 500 parts per million NO is very temperature sensitive. Reducing temperature from 3600.degree. F. to 2800.degree. F. cuts the rate of NO formation 138-fold. A 400.degree. F. drop from 3600.degree. F. to 3200.degree. F. cuts rates by a factor of 9.4. A 400.degree. F. drop from 3200.degree. F. to 2800.degree. F. cuts rates 14.7-fold. A further 400.degree. F. drop from 2800.degree. F. cuts rates by a factor of 84.6. For different oxygen percentages (different air/fuel ratios) the trends of NO formation with temperature are similar. Formation rates also go as the square of species concentrations, and chemical equilibria vary with concentration (pressure) in a way which should be familiar to those who have studied chemical kinetics.
As the fuel/air ratio of a combustible element changes, the NO.sub.x formed from its combustion products will change because changing air/fuel ratio varies peak flame temperature (since it varies the energy of the fuel available to raise the temperature of the fuel and air atoms' mass) and because changing air/fuel ratio changes oxygen available to combine with nitrogen. As will be made clear in the drawings, the temperature effect is typically more important than the oxygen availability effect. So long as an element of combustible mixture is locally homogeneous, its NO.sub.x formation behavior will be straightforwardly described by chemical kinetics calculations. If a mixture of fuel and air is heterogeneous, with many elements at many different air/fuel ratios, the conceptual process of NO.sub.x formation is the same but the arithmetic difficulties of integrating NO.sub.x effects which differ from element to element are formidable.
Complete mixing to molecular scales of fluids is quite difficult, and the difficulty of mixing becomes greater as the geometric scale of the mixer or burner increases. For this reason, the industrial designs which produce combustion on a large scale are heterogeneous. Combustion in electrical power plants is heterogeneous when natural gas is burned, yet more heterogeneous for oil-fired plants, and more heterogeneous still for coal-burning plants. The same can be said for large industrial burners.
Both power plants and large industrial burners are applications where fuel economy is important, and the need to minimize stack heat losses [stack loss equals M C.sub.P (T-T.sub.ambient)] constrains the overall operation of the burners to air/fuel ratios having just enough excess air to complete combustion under the mixing conditions in the burner. Minimum excess air is required to minimize massflow, M, up the stack. NO.sub.x control from these large burners cannot, therefore, involve variations in air/fuel ratio without significant fuel penalties.
The other large sources of NO.sub.x also do not permit variation of air/fuel ratio for NO.sub.x control. Nitric oxide generated in pipelines is generated in the heterogeneous can combustors of stationary turbine engines or in the heterogeneous combustion processes happening in the very large and badly mixed natural gas reciprocating engines used to drive pumps. For both the turbine and reciprocating pump engines, load control is largely achieved by variations in air/fuel ratio. These systems are both so heterogeneous in their mixing processes that they would have many zones producing NO at a maximum rate regardless of the overall air/fuel ratio of operation which might be chosen.
Diesel engines, some of which are mobile and some of which are stationary, also achieve load control by variation in fuel input, and are also inescapably heterogeneous because fuel is sprayed into combustion air only a few milliseconds before combustion initiates, making homogeneity at the level relevant to chemical kinetics impossible.
The inventor has spent the last decade working to control mixing and chemical concentrations to radically reduce NO formation in the flames which occur in spark-fired, internal combustion engines. The work done by the inventor, (largely described in U.S. Pat. No. 4,344,934), shows that complete microscale and large-scale homogeneity of fuel, air and residual gas produces radical (as much as 1,000-fold) reductions in NO.sub.x. These NO.sub.x reductions fit closely the theoretical predictions of chemical kinetics. It should be emphasized that the physical scale on which combustion and the NO forming chemical reactions occur is of the order of molecular mean free paths or at most localities in the size range of a few cubic microns. The chemistry occuring in these tiny volumes should not vary with the container in which these tiny volumes occur. There is every reason to believe that the chemical kinetics of NO formation is as valid in a hundred megawatt power plant as in a small reciprocating piston engine. If the mixing state of the individual elements which burn can be well-described, kinetics should predict accurately NO formation rates.
NO.sub.x formation from non-mobile sources is dominated by heterogeneous combustion process. Fuel/air ratio varies radically from place to place within the burner, and temperature-pressure-time trajectories which determine NO formation vary accordingly. However, it is still possible to radically control NO formation in these heterogeneous combustion processes.
The approach is simply stated. If some of the products of combustion, after doing work on turbines or passing through heat exchanger tubes, are recirculated into the intake air and perfectly mixed with this air prior to introduction of the EGR-air mixture to the burner, peak flame temperatures in the burner will be lowered for every element of fuel/air mixture burned because every element of fuel/air mixture will have the same ratio of diluent to air, and this EGR diluent, because of its specific heat and mass, will suppress peak flame temperatures. This lowering of peak combustion temperatures will occur in every element to be burned regardless of the details of the fuel-oxidizer mixing process in the burner. Because NO formation rates are temperature sensitive, this lowering of combustion temperatures will reduce NO output. The magnitude of the NO reduction can be very large.
It should be noted that exhaust gas recirculation percentage does not change the stack losses and efficiency of a powerplant or large burner so long as stack gas temperature does not change, since EGR does not change mass flow out the stack, M. EGR is also compatible with the operation of large reciprocating piston natural gas engines, turbine combustors, and diesels. From a combustion point of view, the better mixed the EGR is with the rest of the air, the more EGR can be tolerated.
The idea of exhaust gas recirculation for NO.sub.x control is not new. EGR equipment has been installed in power plants, stationary burners, stationary turbines and large reciprocating natural gas engines pumping for natural gas pipelines, and diesel engines for many years. However, the NO.sub.x suppression results achieved with these devices have been radically less than those which could have been achieved with the same exhaust recycle percentages if the EGR was homogeneously mixed with the rest of the air.
The theoretical advantages of EGR have been long known. In the figures, kinetics calculation results from Bartok (Exxon Research) are shown indicating a 90 percent NO.sub.x reduction, with 10 percent exhaust gas recirculation, for a homogeneous combustor case. The same sort of calculation would indicate a 98 percent reduction for 20 percent exhaust recirculation. However, 20 percent exhaust gas recirculation, mixed conventionally, has been used in many power plant burners. In these burners, the EGR only cuts NO.sub.x output in half.
The disparity between NO.sub.x reductions available in theory and those obtained in large scale burners is due to bad EGR-air mixing. However, efforts to control NO.sub.x with EGR have taken low priority in industry, and extremely expensive schemes for catalytic reduction of NO.sub.x, some of them involving capital expenditures of billions of dollars, are being actively pursued. This is happening because the vital important of EGR-air mixing to the NO.sub.x reductions obtained has not been understood.
In addition, mixing fluid mechanics is a difficult business, and mixing sections capable of producing the required large-scale, middle-scale and micro-scale homogeneity of air and EGR have not been available. The efforts of the inventor to produce practical and rapid mixing using controlled flow structures make it possible now to build such mixing sections practically. The basic structured turbulent flow mixing process has been worked out and tested on reciprocating spark-fired engines, but can be scaled-up readily to the sizes required for large stationary burners.
It is important to realize how difficult complete mixing is, and how unsatisfactory conventional mixing techniques are in industry. Moreover, it is important to realize that the current state of engineering and fluid mechanical knowledge concerning the detailed processes required to achieve complete mixing is in a primitive state. Insights into the structure of turbulence and recognition of the importance of the problme now make mixing an important and accessable research problem in fluid mechanics. For instance, a team of researchers including professors S. J. Kline, B. J. Cantwell, and L. Heselink at Stanford University are now (1983) initiating a major effort to study mixing in jets and in structured flow sections such as the one worked out by the inventor.